Hmm... well what I have been doing is using awebar. Like you I have a series of 6 emails sent at a set interval.
When I have dispatched the DVD, I manually add the email address to the autoresponder (no confirmation required) and then they get the 6 emails.
The best thing about this is if it got to a stage where someone has received all of the 6 mails then you still have there email address sitting there in the autoresponder.
This is great for when there is a Lotto or Euromillions rollover... you can then send the active people in the list an email about the rollover.
2 other things:-
If after the 6 emails they still havent signed up you could always just post them a letter (just incase they didnt receive the emails) asking them whether they received it bla bla bla
Also when someone requests a DVD its a good idea just to send a few short lines in an email along the lines of "We have received your request and are just confirming that you did actually make the request and we have the correct details for you" etc
If they respond... send it - if they dont then its up to you.